Permanganate ion color

Oxidizing agents also attack the double bond. When a reaction between an unsaturated compound and the permanganate ion occurs, the violet color of permanganate fades. This reaction, as well as reaction (29b) in which a color change also occurs, is used as a qualitative test for the presence of double bonds in compounds. 

The stoichiometric point is reached when all the Fe2+ has reacted and is detected when the purple color of the permanganate ion persists. A sample of ore of mass 0.202 g was dissolved in hydrochloric acid, and the resulting solution needed 16.7 mL of 0.0108 M KMn04(aq) to reach the stoichiometric point, (a) What mass of iron(ll) ions is present (b) What is the mass percentage of iron in the ore sample ... 

The colors that we have described arise from d-d transitions, in which an electron is excited from one d-orbital into another. In a charge-transfer transition an electron is excited from a ligand onto the metal atom or vice versa. Charge-transfer transitions are often very intense and are the most common cause of the familiar colors of d-metal complexes, such as the transition responsible for the deep purple of permanganate ions, Mn04 (Fig. 16.33). 

The purple color of the permanganate ion slowly disappears as the reaction proceeds. 

Elemental composition K 24.74%, Mn 34.76%, 0 40.50%. The compound may be identified by its dark purple color and other physical properties, and confirmed by chemical analysis for the elements potassium and manganese by AA, ICP, and other instrumental means (see Potassium and Manganese). The concentration of permanganate ion, Mn04 in aqueous solution may be determined by titration with a standard solution of oxahc acid or ferrous ion (see Reactions). 

Addition of aqueous Br to a solution of Mn04 causes a reduction of the permanganate ion, discharging the intense purple color. 

The thiovanadate ion, VS73, resembles the permanganate ion in color. Like the latter, it is one of the few deeply colored ions known in which there are no partially-filled d subshells. 

A potassium permanganate solution is standardized by dissolving 0.9234 g sodium oxalate in dilute sulfuric acid and then titrating with the potassium permanganate solution. The principal products of the reaction are man-ganese(II) ion and carbon dioxide gas. The titration requires 18.55 mL of the potassium permanganate solution to reach the endpoint, which is indicated by the first permanent, but barely perceptible, pink color of the permanganate ion. 

The amount of manganese in steel is determined by changing it to permanganate ion. The steel is first dissolved in nitric acid, producing Mn2+ ions. These ions are then oxidized to the deeply colored Mn04 ions by periodate ion (I04 ) in acid solution. 

Manganese can exist in all oxidation states from +2 to +7, although +2 and +7 are the most common. Manganese(II) forms an extensive series of salts with all the common anions. In aqueous solution Mn2+ forms Mn(H20)s2+, which has a light pink color. Manganese(VII) is found in the intensely purple permanganate ion (Mn04 ). Widely used as an analytical... 

FIGURE 11.9 (a) Addition of a small amount of potassium permanganate from a buret gives a dash of purple color to the Fe solution, which disappears quickly as the permanganate ion reacts to give Fe and Mn. (b) When all the Fe ions have been consumed, additional drops of permanganate solution give a pale purple color to the solution. 

The reaction between permanganate ion and oxalic acid is complex and proceeds slowly even at elevated temperature unless manganese(II) is present as a catalyst. Thus, when the first few milliliters of standard permanganate are added to a hot solution of oxalic acid, several seconds are required before the color of the permanganate ion disappears. As the concentration of manganese(ll) builds up, however. the reaction proceeds more and more rapidly as a result of autocatalysis. 

As an example, let s consider the reaction between permanganate ion (Mn04 ) and oxalate ion (C204 ) in acidic aqueous solution (T FIGURE 20.2). When Mn04 is added to an acidified solution of C204, the deep purple color of the Mn04 ion fades, bubbles of CO2 form, and the solution takes on the pale pink color of Mn. We can therefore write the skeleton equation as... 

In the laboratory portion of your course, you have probably seen many colorful transition-metal compounds, including those shown in T FIGURE 23.35. Many of these compounds are colored he- - cause of d-d transitions. Some colored complexes, however, including the violet permanganate ion, Mn04, and the yellow chromate ion, Cr04 , derive their color from a different type of excitation involving the d orbitals. 

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